The invention relates generally to an improved sheet feeding device and, more specifically, to a sheet feeding device used in offset or sheet feed printers for separating single sheets from a stack of sheets.
High-speed offset or sheet feed printers, such as the HEIDELBERG SPEEDMASTER.RTM. Multicolor Press, the HARRIS.RTM.Press, the KOMORI.RTM.Press, the MIEHLE.RTM.Press and the MAN-ROLAND.RTM.Press, include a reciprocating lifting means, such as a suction device, for lifting a single sheet of paper from a hopper or bin located on the outside portion of the printer, and transferring that sheet to the interior printing section of the printer. However, electrostatically charged paper which tends to stick together often impedes the efforts of the lifting device to select only one sheet of paper at a time per operation from the stack. For this reason, a secondary sheet feeding device has been found necessary to ensure that only one sheet, instead of two or more sheets, is conveyed to the printing means of the printer.
Such an individual sheet feeding apparatus has a variety of benefits. High-speed printing is expensive, and thus extremely costly to the owner when printer operation must temporarily cease due to improper multi-sheet feeding. Moreover, modern high-speed printers maintain precisely set water/ink balances. Sporadic downtime of such printers causes these preset color ratios to be upset, requiring further operator labor and time to reassess and reset the color ratios.
In many prior art printing applications, especially high speed printing applications, high-mass gripper jaw assemblies have been utilized to grasp sheets and pull them from stacks. See, for example, U.S. Pat. No. 4,013,283. However, such gripper jaw assemblies are relatively complicated and tend to experience vibrational and other problems at higher feeding speeds.
Other sheet feeding apparatus have included paper separating devices which are adapted to exert a downward pressure on the top sheet of a stack, thus ensuring that the reciprocating lifting means, such as an air suction device, only delivers one sheet per operation to the feeder and printing means of the press. FIGS. 6 and 7 illustrate two such prior art devices. These devices utilize one stationary metal spring, which is manually adjustable in response to a situational need. However, such prior art devices do not have the necessary flexibility to allow sheets of widely varying thickness to be individually selected. Moreover, such prior art devices do not have the capability of allowing individual springs to be adjusted or fanned out in a number of different positions, depending upon the type or thickness of paper being used.
It is becoming even more important to provide a device which will separate sheets for feeding. Standard paper used in the printing industry has a dimension of 28 wide inches by 40 inches in length. However, new high speed printers now have the capability of accommodating paper sizes of up to 56 inches in length. This increased surface area gives electrostatically charged paper enhanced sticking power.
It is therefore desireable to provide a relatively inexpensive secondary sheet feeding apparatus which would cooperate with a primary sheet feeding apparatus (including lifting means such as an air suction device), and eliminate the problems associated with the prior art devices shown in FIGS. 6 and 7, while providing the desired advantages outlined above, as well as other advantages which will become apparent from reading the more detailed description of the preferred embodiment, below.